You can rest assured a company firing you for what they saw while surveilling your work computer will not be so stupid as to reveil this fact. That would indeed be a liability for them. They will simply invent a different reason for firing.

Because they know it's not allowed (or at least frowned upon), but they decided to do it anyways, the company surveillance is kept secret and downplayed and plausibly denied as much as possible.

I think the keyword is “can”.

It is allowed, contrary to eg the EU, where this is not allowed.

I always assume it is the case that my company will spy on my work computer. It’s naive to assume otherwise; there are just too many incentives/externalities for it to not happen given enough time and a reasonably funded infosec department.

if it's a device provided by your company, it's very likely it'll have some spyware on it.